1. Field of the Invention
This invention relates generally to Switched Reluctance Machines (SRMs) and, more particularly, to a system and method for inductance based position encoding for sensorless SRM drives.
2. Background Art
Switched reluctance machine (SRM) drives have been considered as a possible alternative to conventional drives in several variable speed drive applications because of the many advantages associated with SRM systems. The performance of an SRM drive can be tailored to suit several applications through appropriate control. Other advantages include constructional simplicity of the machine such as, for example, the absence of permanent magnets and windings in the rotor; fault tolerant operation of the inverter; an extended high speed operating range; and the like. Recent literature indicates that SRM drives are suitable for electric vehicles, electric traction applications, home appliances, consumer applications, automotive applications, power steering application in vehicles, aircraft starter/generator systems, and the like.
Rotor position sensing is an integral part of SRM control due to the nature of torque production. Sensorless control reduces overall cost and dimension of the drive in addition to improving reliability. Also, there are certain applications, such as in compressors, where the ambient conditions do not allow the usage of external position sensors.
A switched reluctance machine, whether functioning as a motor or a generator, is basically a doubly salient, singly-excited machine that operates on the basis of a reluctance torque generation principle. Each stator phase is excited with pulses of active currents during the positive inductance slope region in order to develop positive unidirectional torque. This requires synchronization of the stator phase excitation with rotor position. Usually, external mechanical position sensors such as resolvers or optical encoders are used. However, these sensors are expensive and experience reliability problems. Various sensorless techniques have been published in the literature which mainly use terminal measurements or diagnostic signals to infer the rotor position.
Several sensorless control methods have been reported. These methods can be broadly classified as signal injection methods, state observer methods, flux integration methods, signal power measurement methods, and the like. Each of the various methods suggested have merits and demerits depending on the principles of operation. Ideally, a sensorless scheme which uses only terminal measurements and does not require additional hardware is preferred.